Manufacture of metal foil



1967 B. J. BLISS 3,335,000

MANUFACTURE OF METAL FOIL Filed OC(,. 4, 1965 F l G. l. 7

United States Patent 3,335,000 MANUFACTURE OF METAL FOIL Bruce J. Bliss, North Attlehoro, Mass., assignor to Texas Instruments Incorporated, Dallas, Tex., a corporation of Delaware Filed Oct. 4, 1965, Ser. No. 492,817 4 Claims. (Cl. 75208) ABSTRACT OF THE DISCLOSURE A flexible supporting strip is coated with a slurry composed of metal powder, a high-molecular-weight binder and a carrier liquid. The viscosity of the slurry is made such that when applied to the supporting strip it will evenly cling thereto. The strip is resistance-heated as it is caused to move through and out ofa supply of the slurry. Thus the strip is coated and the slurry dried on the strip after emergence from the supply. Then the dried slurry is compressed on the strip to green-bond the particles of powder to one another in the solid phase. The dried green-bonded and compressed slurry, which at this time remains flexible, is peeled from the strip. It is then sintered at a temperature to drive off the dried binder and at the same time promote grain growth between the green-bonded powder particles. This grain growth increases in strength the solid-phase bonds between the particles and obliterates porosity in the resulting foil. The solid and substantially nonporous foil is finally rolled to desired smoothness and gauge.

Among the several objects of the invention may be noted the provision of an improved method and apparatus for manufacturing comparatively long continuous lengths of thin gauge metal foil of the desired thickness directly from metal powders without the need for initially compacting deep deposits of dry powders; the provision of a convenient method and apparatus for manufacturing metal foil from a slurry containing particles of the metal; the provision of a method and apparatus for manufacturing metal foil from metal powder wherein the resulting foil is substantially pore-free; the provision of a method for manufacturing metal foil wherein'two lengths of the foil may be simultaneously processed in a part of the method; and the provision of a method for manufacturing metal foil from a slurry wherein a strip for carrying the slurry during manufacture of the foil may be reused. Other objects and features will be in part apparent and in part pointed out hereinafter.

The invention accordingly comprises the methods, apparatus and products hereinafter described, the scope of the invention being indicated in the following claims.

In the accompanying drawings, in which one of various possible embodiments of the invention is illustrated,

FIG. 1 is a diagrammatic view, parts being broken away, illustrating steps in the manufacture of metal foil in accordance with the invention; and

FIG. 2 is a diagrammatic view showing a subsequent step employed.

Corresponding reference characters indicate corresponding parts throughout the several views of the drawings.

It is known to manufacture metal foil by rolling down material from metal ingots. This is costly. Metal foil has also been produced in two stage processes from metal powders by first compacting substantially thick layers of powders, usually in short lengths or small areas, and then rolling the powder compaction to foil thickness (of, for example, 0.005" or so). The need to compact the thickly laid powders introduces difiiculties and slows production. The present invention is for an economical process for 3,335,000 Patented Aug. 8, 1967 Ice manufacturing comparatively long and continuous lengths of thin dense strips or foil of metal from thin layers of the powders of the metal near the final gauge desired.

In the following description, reference to metals used is intended to include alloys thereof, including, but without limitation, the metals nickel, copper, aluminum, iron, tin, et cetera. Foil refers to thin sheet metal, the thickness being a few thousandths of an inch, usually about 0.005" or less. The term slurry means a liquid medium of substantial viscosity containing metal particles suspended in a binder. The term binder means long-chain, high-molecular-weight organic compounds or the like characterized in that their constituents when comminuted are stringy and when mixed with a liquid such as water swell and act according to the invention to hold or bind the metal particles in suspension and to produce adequate viscosity in the slurry. Thus the slurry will adhere evenly to metal surfaces contacted by it. Examples, but without limitation, of suitable binders are methyl cellulose, nonionic cellulose ether, polyethylene oxide, polyvinyl pyrrolidone et cetera. The intended use of the foil will determine the particular gauge to which it is manufactured. All drawings are illustrative and not to scale because of the small dimensions involved.

Briefly, the process for manufacturing foil according to the invention comprises coating a flexible preferably metal supporting or carrier strip with a slurry containing a metal powder suspended in a binder, drying the slurry by heating, and then compacting and sintering it. The slurry includes a mixture of a long-chain high-molecularweight organic compound acting as a binder, a carrier such as water, and a powder of the metal desired for the foil, the powder being of small average particle size of less than 50 microns with 5 microns preferred and with substantially no particles exceeding 100 microns in size. The particles are evenly suspended by the binder. The amount of the binder used controls the viscosity of the slurry so that when applied to a metal surface it will cling thereto. When evenly applied, the slurry will remain substantially even in any position of the metal surface. In

some but not all cases a conventional wetting agent such as an aerosol may be used to advantage in the slurry. The total solid content of the slurry is preferably high and desirably exceeds 50% by weight of the total weight of the slurry.

When the slurry has been thoroughly mixed it is, for example, poured into a tank such as illustrated at 1 in FIG. 1. Tank 1 has a pair of sealing gaskets 3 at its lower end. It is generally V-shaped and open at the top as illustrated. The slurry, designated 5, may be agitated or circulated by a stirrer, illustrated diagrammatically at 2, to maintain the metal particles and liquid portions of the slurry in a thoroughly mixed or homogeneous state. A flexible carrier strip 7 is unwound from a reel 9 and passed upwardly through the seals 3 and out of the open top of the tank 1. It is subsequently wound on a reel designated 11. The carrier strip 7 is preferably in the form of a hard metallic strip such as cold rolled stainless steel but if desired may be made of other appropriate metal.

As strip 7 is drawn through the slurry 5 both of its surfaces pick up and become uniformly coated with the high-viscosity slurry 5. These wet coatings are designated 13 and 15 in FIG. 1. They are immediately dried. As dried they are lettered 14 and 16. The means illustrated in the drawings for drying the wet coatings 13 and 15 comprise resistance heating of the strip 7 by connecting a source of electric current 17 to the strip 7 as shown. The electrical resistance of strip 7 results in heat being conducted to the wet coatings 13 and 15 which as they move reach the dry condition 14 and 16. While the thick- 3 ness of coatings 13 and 15 may vary, a dried thickness at 14 and 16 of about 0.006" for each coating has been found satisfactory for producing a final foil thickness of about 0.002".

Strip 7 with the dried coatings 14 and 16 thereon is then run through rolls of a conventional rolling mill, designated 19. Rolls 19 compact the coatings 14 and 16, thereby reducing their thickness and increasing their density in the aggregate as indicated at 18 and 20. Preferably the rolls of the mill effect a reduction in the dry coating thickness of about 50% or more. Thus a thickness of about .0025 to 0.003" obtained from an original thickness of about 0.006 is satisfactory. As the dry coatings 14 and 16 are compacted in the rolling mill 19, adjacent particles of the metal in the coating become green-bonded to each other in the solid phase, thereby providing a suflficiently cohesive metallic structure to be handled in the remainder of the process. However, the particles do not substantially green-bond to the strip 7 because of the particle size, shape and hardness. It will be understood that the pressure required for the requisite reduction of the dried slurry is preferably not such as to reduce the thickness of the strip 7, so that it may be reused.

The coatings 14 and 16, when dried and compacted by reduction as at 18 and 20, are peeled from strip 7 and rolled onto reels 21 and 23, respectively. The binder in the dried slurry is of a nature to permit the peeled strips 18 and 20 to flex for coiling immediately after the compaction step. The carrier strip 7 is then wound on reel 11 and, since the coating has not bonded to the carrier strip during compaction by the rolling mill 19, the carrier strip can be reused with little or no cleaning or other work being performed on it. Stripping of the compacted dried coatings 18 and 20 from the carrier strip 7 is facilitated by a naturally occurring curling phenomna sometimes called alligatoring. This is the tendency of two sheets being rolled to curl away from one another at the roll exit. Thus the dry coating 18 tends to curl to the left as viewed in FIG. 1 while the dry coating 20 tends to curl to the right, thus facilitating drawing away of the coatings from the carrier strip by the reels 21 and 23.

Next the material on reels 21 and 23 is sintered at a time and temperature sufficient to remove by vaporization the dried binder in the coating, and by diffusion and grain growth to increase the solid-phase bonds between the metal grains in the strips 18 and 20. The resulting foil strip is very dense and substantially pore-free. The sintering furnace may contain an inert protective or a reducing atmosphere such as argon, hydrogen or the like, depending upon the nature of the metal constituting the starting powdered metal.

Since the sintering step leaves the foil soft and with a somewhat rough surface, the resulting reel of foil may be further treated to harden it and produce a smoother surface. This step is illustrated in FIG. 2 wherein a reel 21 carries material such as say 18 after sintering. This is unwound and passed through squeeze rolls 25 where the material is very lightly compressed by means of a so-called kiss pass which reduces the thickness of the foil from about 0.0025" to about 0.0022. The foil is then rolled up on a reel shown at 27.

The following is a particular example of how the process of the invention has been performed:

A 2.6% mix of 400 centipoise grade methyl cellulose in water was diluted with additional water to a viscosity of about 1500 centipoises. To 350 cc. of this material was added 575 grams of nickel powder having an average particle size of about 5 microns. To this was added 30 grams of a aerosol mix, the latter being a known wetting agent. The total solid content of the resulting slurry became 60%. The slurry was thoroughly mixed and poured into a tank such as illustrated at 1 in FIG. 1. The carrier strip 7 used was a length of hard stainless 4 steel which had been cold-rolled from a thickness of about 0.075" to about 0.010.

The carrier strip 7 was uniformly coated on both sides as shown at 13, 15 by passing it up through the slurry 5, and after drying by resistance heating the coating at 14, 16 had a thickness of about 0.006". The coated strip was then run through pressure rolls such as 19 where the thickness of the dried coatings 14, 16 was reduced about 50%. The resulting compaction of the coating as illustrated at 18, 20 was accomplished without the rolls 19 touching the steel strip 7 and without causing reduction in its thickness. As a result, adjacent nickel particles in the compacted coatings were green-bonded to each other but there was substantially no bonding of the nickel to the steel carrier strip. The reason for this was the fineness of the nickel particles, which was 5 microns. A range of 2 to 14 microns is, however, useful to obtain adequate green bonding between particles without green-bonding to the backing strip. Then the foil was peeled from the carrier strip, coiled and placed in a furnace for sintering at about 1800 F. for approximately 30 minutes. The furnace contained a dry hydrogen reducing atmosphere to reduce any oxidation present. Sintering in the furnace removed the methyl cellulose binder and promoted grain growth between the green-bonded metal particles to increase strength. The resulting foil, although soft and having a somewhat rough surface, was very dense, substantially pore-free and had a thickness of about 0.0025". The foil was then passed through finishing rolls as shown at 25 in FIG. 2 where it was given a kiss pass to reduce its thickness to a uniform thickness of 0.0022". The foil had a bright finish equal to nickel strip produced by other processes.

It will be understood that any of the materials mentioned above may be employed in the process outlined. It may be mentioned that the use of the wetting agent in the slurry is not always required. Thus it is not required when polyethylene oxide, nonionic cellulose ether or polyvinyl pyrrolidone are used as the long-chain, high-molecular-weight organic compound to form the slurry. Also, while in the example the coatings 13 and 15 are applied to strip 7 by passing the strip up through tank 1, it will be understood that the coatings can be applied to one or both surfaces of the strip by a roller coater, brushing or by other suitable means for applying a uniform coating on the carrier strip. While the strip 13 is shown as being reeled prior to passing it through the furnace, it will be understood that the strip can be sent through a sintering furnace directly from the rolls 19, and then directly to the finishing rolls 25.

In view of the above, it Will be seen that the several objects of the invention are achieved and other advantageous results attained.

As various changes could be made in the above methods without departing from the scope of invention, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is: 1. The method of manufacturing thin solid and substantially pore-free metal foil, comprising coating on at least one side of a supporting carrier a layer of a slurry containing a binding compound, a liquid and powdered metal,

drying the layer of slurry on the carrier,

squeezing the dried layer on the carrier to reduce the thickness of the layer and green-bond the particles of powdered metal,

removing the dried and squeezed layer from the supporting carrier, and

sintering the removed layer to drive off the binding compound and by further grain growth substantially to eliminate pores and improve the green bonds between said particles.

2. The method of manufacturing thin solid and substantially pore-free metal foil, comprising making a slurry containing a high-viscosity binding compound, a liquid and powdered metal,

coating the slurry on both sides of a carrier strip,

drying the adhered coatings,

roll-squeezing the dried coatings from opposite sides of the carrier strip to reduce the thicknesses of the coatings and green-bond the particles of powdered metal to one another,

removing the coatings from the carrier strip, and

sintering the stripped coatings to drive off the binding compound and by grain growth substantially eliminate pores and improve the green bonds between the particles.

3. The method of manufacturing thin solid and substantially pore-free metal foil, comprising establishing a volume of slurry containing a binding compound, a liquid and a powdered metal,

moving a metal carrier strip through the volume of slurry to emerge therefrom and coat layers thereof on opposite sides of the strip,

passing an electric current through the strip to dry the layers upon movement of the strip from said volume,

roll-squeezing the dried layers on the strip to reduce References Cited UNITED STATES PATENTS 2,582,744 1/1952 Brennan. 2,851,354 9/1958 Scanlan 222 2,900,254 8/1959 Raiklen 75--2l4 3,121,631 2/1964 Comstock 752l4 3,227,591 1/1966 Lambert 75214 X FOREIGN PATENTS 855,203 11/ 1960 Great Britain.

CARL D. QUARFORTH, Primary Examiner.

BENJAMIN R. PADGETT, Examiner.

A. I. STEINER, Assistant Examiner. 

1. THE METHOD OF MANUFACTURING THIN SOLID AND SUBSTANTIALLY PORE-FREE METAL FOIL, COMPRISING COATING ON AT LEAST ONE SIDE OF A SUPPORTING CARRIER A LAYER OF A SLURRY CONTAINING A BINDING COMPOUND, A LIQUID AND POWDERED METAL, DRYING THE LAYER OF SLURRY ON THE CARRIER, SQUEEZING THE DRIED LAYER ON THE CARRIER TO REDUCE THE THICKNESS OF THE LAYER AND GREEN-BOND THE PARTICLES OF POWDERED METAL, REMOVING THE DRIED AND SQUEEZED LAYER FROM THE SUPPORTING CARRIER, AND SINTERING THE REMOVED LAYER TO DRIVE OFF THE BINDING COMPOUND AND BY FURTHER GAIN GROWTH SUBSTANTIALLY TO ELIMINATE PORES AND IMPROVE THE GREEN BONDS BETWEEN SAID PARTICLES. 